a) Field of the Invention
The present invention relates generally to a semiconductor light emitting device and a method of manufacturing the same, and particularly to a porous semiconductor light emitting device and a method of manufacturing the same, in which a radiative mechanism is obtained through the use of porous fine wires.
Silicon (Si) light emitting devices which have previously been considered impossible to manufacture are now being realized through the provision of a porous region formed in Si using hydrofluoric acid. Si light emitting devices alleviate problems encountered in the implementation of photoelectric integrated circuits on Si wafers, and open the way to optical coupling between wafers and to other technologies.
b) Description of the Related Art
Silicon has indirect band gaps, and is generally not considered to be suitable for use in preparing light emitting devices. For this reason, materials such as GaAs, InP or the like having direct band gaps have been used conventionally in constructing such devices.
It has been discovered, however, that Si provides a radiative mechanism when it undergoes anodic oxidation in an aqueous solution of hydrofluoric acid whereby a thin wire porous structure is formed on the surface thereof. It has been found that when an exciting light is applied to a porous silicon structure having poles which are 10 nm or less in diameter, light is emitted in the order of 1.4 to 1.6 eV (L. T. Canham, Appl. Phys. Lett., 57(10), 1046 (1990), V. Lehmann and U. Gosele, Appl. Phys. Lett., 58(8), 856 (1991), N. Koshida and H. Koyama, Jpn. J. Appl. Phys., 30(7B), L1221 (1991)).
It is believed that this phenomenon results from pseudo direct transition between bands because of sub bands of a limited size formed in the silicon region.
The band gap E allowing recombination transition is defined by the relationship (h.sup.2 /4m.sub.e.sup.* d.sup.2 +h.sup.2 /em.sub.h.sup.* d.sup.2) where d is the width of a potential well, h is the Planck constant, and m.sub.e.sup.* and m.sub.h.sup.* are the effective masses of electron and hole. The wavelength of emitted light becomes shorter as the width of a potential well becomes narrower. The radiative phenomenon may result from the quantum effect of the miniaturization of the silicon region to the size of about 5 nm or less.
One method of making a super fine silicon region, is to form the Si into fine particles. This method is associated, however, with particle distortion, thus lowering the light emission efficiency. It is also difficult to inject carriers into fine particles.
Another method of making a super fine silicon region is the method proposed by L. T. Canham, whereby the silicon region is made porous by way of anodic oxidation (L. T. Canham, Appl. Phys. Lett., 57(10), 1046 (1990)).
In the anodic oxidation procedure, a Si wafer with an electrode formed on the back surface thereof is immersed in an aqueous solution of HF. A d.c. power source is connected between the Si wafer electrode and a counter metal electrode which also is immersed in the HF solution.
The Si wafer electrode is used as the anode, and the counter metal electrode is used as the cathode. Under these conditions, anodic oxidation occurs on the front surface of the Si wafer. The oxide film formed by anodic oxidation immediately dissolves in the HF aqueous solution. Holes are essential for the anodic oxidation process. A p-type Si is therefore used, or electron-hole pairs are generated by applying light to a Si wafer.
The surface of the Si wafer in the HF aqueous solution is etched by anodic oxidation, and fine pores or recesses are formed at local areas where the etching speed is great. Once recesses are formed, the electric field concentrates at these local areas so that holes are produced from the inside of the bulk to allow selective etching. Since the electric field concentrates at the bottom areas of the recesses having a large radius of curvature, the etched recesses do not extend in the lateral direction but extend generally in the depth direction.
In the above manner, a porous region is formed at the Si wafer surface. The porosity depends on the HF concentration of the HG aqueous solution, the electric power, the carrier concentration, and other factors. When the porosity exceeds 50%, the recesses become interconnected and a cluster of Si wires having fine poles is formed. In this manner, Si quantum wires having a diameter of 5 nm or less are obtained to thereby present the quantum effect.